Timing adjustment for synchronous operation in a wireless network

ABSTRACT

Techniques for adjusting transmit timing of base stations and user equipments (UEs) in a wireless network are described. In one operating scenario, a femto base station communicates with a femto UE, and a macro base station communicates with a macro UE located within the coverage of the femto base station. In an aspect, the transmit timing of the femto base station may be delayed relative to the transmit timing of the macro base station, e.g., to time align downlink signals from the femto and macro base stations at the femto and macro UEs. In another aspect, the transmit timing of the femto UE may be advanced relative to the transmit timing of femto base station by an amount larger than twice the propagation delay between the femto UE and the femto base station, e.g., to time align uplink signals from the femto and macro UEs at the femto base station.

The present application for patent is a divisional of patent applicationSer. No. 12/712,755, entitled “TIMING ADJUSTMENT FOR SYNCHRONOUSOPERATION IN A WIRELESS NETWORK,” filed Feb. 25, 2010, allowed, whichclaims claims priority to provisional U.S. Application Ser. No.61/156,816, entitled “A METHOD AND APPARATUS FOR UPLINK AND DOWNLINKTIMING ADJUSTMENT FOR LOW POWER CELLS,” filed Mar. 2, 2009, assigned tothe assignee hereof and hereby expressly incorporated herein byreference.

BACKGROUND

I. Field

The present disclosure relates generally to communication, and morespecifically to techniques for supporting communication in a wirelesscommunication network.

II. Background

Wireless communication networks are widely deployed to provide variouscommunication content such as voice, video, packet data, messaging,broadcast, etc. These wireless networks may be multiple-access networkscapable of supporting multiple users by sharing the available networkresources. Examples of such multiple-access networks include CodeDivision Multiple Access (CDMA) networks, Time Division Multiple Access(TDMA) networks, Frequency Division Multiple Access (FDMA) networks,Orthogonal FDMA (OFDMA) networks, and Single-Carrier FDMA (SC-FDMA)networks.

A wireless communication network may include a number of base stationsthat can support communication for a number of user equipments (UEs). AUE may communicate with a base station via the downlink and uplink. Thedownlink (or forward link) refers to the communication link from thebase station to the UE, and the uplink (or reverse link) refers to thecommunication link from the UE to the base station. The wireless networkmay support synchronous operation on the downlink and uplink. It may bedesirable to properly adjust transmit timing of base stations and UEs inorder to achieve good performance.

SUMMARY

Techniques for adjusting transmit timing of base stations and UEs in awireless network are described herein. In one operating scenario, afemto base station may communicate with a femto UE, and both may belocated within the coverage of a macro base station. A macro UE maycommunicate with the macro base station and may be located within thecoverage of the femto base station. The different types of base stationsand UEs are described below. The femto UE and the macro UE may observehigh interference due to their close proximity. The high interferencemay be mitigated with proper transmit timing for the femto base stationand the femto UE.

In an aspect, the transmit timing of the femto base station may bedelayed relative to the transmit timing of the macro base station. Inone design, the femto base station may determine the propagation delaybetween the macro base station and the macro UE, or the femto UE, or thefemto base station. The femto base station may determine an amount ofdelay for its transmit timing based on the propagation delay. The femtobase station may determine a reference time (e.g., GPS time) used forthe transmit timing of the macro base station. The femto base stationmay then delay its transmit timing by the determined amount of delayfrom the reference time. The femto base station may also adjust itstransmit timing in other manners, as described below. In one design, thetransmit timing of the femto base station may be adjusted to time aligndownlink signals from the femto and macro base stations at the femto andmacro UEs.

In another aspect, the transmit timing of the femto UE may be advancedrelative to the transmit timing of femto base station by an amountlarger than twice the propagation delay between the femto UE and thefemto base station. In one design, the femto base station may determinea macro propagation delay, which may be the propagation delay betweenthe macro base station and the macro UE, or the femto UE, or the femtobase station. The femto base station may determine an amount of advancefor the transmit timing of the femto UE based on the macro propagationdelay. The femto base station may then advance the transmit timing ofthe femto UE by the determined amount of advance from the referencetime. The transmit timing of the femto UE may be advanced by twice themacro propagation delay relative to the transmit timing of the femtobase station. The transmit timing of the femto UE may also be adjustedin other manners, as described below. In one design, the transmit timingof the femto UE may be adjusted to time align uplink signals from thefemto UE and the macro UE at the femto base station.

The techniques described herein may also be used for other types of basestations and UEs. Various aspects and features of the disclosure aredescribed in further detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication network.

FIG. 2 shows an exemplary frame structure.

FIG. 3 shows an operating scenario with two base stations and two UEs.

FIG. 4 shows a timing diagram of downlink and uplink transmissions fromthe base stations and UEs in FIG. 3.

FIG. 5 shows a timing diagram for a femto base station and a femto UE.

FIGS. 6 and 7 show processes for adjusting transmit timing of a basestation.

FIG. 8 shows an apparatus for adjusting transmit timing of a basestation.

FIGS. 9 and 10 show processes for adjusting transmit timing of a UE.

FIG. 11 shows an apparatus for adjusting transmit timing of a UE.

FIG. 12 shows a process for communication by a UE.

FIG. 13 shows an apparatus for communication by a UE.

FIG. 14 shows a block diagram of a base station and a UE.

DETAILED DESCRIPTION

The techniques described herein may be used for various wirelesscommunication networks such as CDMA, TDMA, FDMA, OFDMA, SC-FDMA andother networks. The terms “network” and “system” are often usedinterchangeably. A CDMA network may implement a radio technology such asUniversal Terrestrial Radio Access (UTRA), cdma2000, etc. UTRA includesWideband CDMA (WCDMA) and other variants of CDMA. cdma2000 coversIS-2000, IS-95 and IS-856 standards. A TDMA network may implement aradio technology such as Global System for Mobile Communications (GSM).An OFDMA network may implement a radio technology such as Evolved UTRA(E-UTRA), Ultra Mobile Broadband (UMB), IEEE 802.11 (Wi-Fi), IEEE 802.16(WiMAX), IEEE 802.20, Flash-OFDM®, etc. UTRA and E-UTRA are part ofUniversal Mobile Telecommunication System (UMTS). 3GPP Long TermEvolution (LTE) and LTE-Advanced (LTE-A) are new releases of UMTS thatuse E-UTRA. UTRA, E-UTRA, UMTS, LTE, LTE-A and GSM are described indocuments from an organization named “3rd Generation PartnershipProject” (3GPP). cdma2000 and UMB are described in documents from anorganization named “3rd Generation Partnership Project 2” (3GPP2). Thetechniques described herein may be used for the wireless networks andradio technologies mentioned above as well as other wireless networksand radio technologies. For clarity, certain aspects of the techniquesare described below for LTE, and LTE terminology is used in much of thedescription below.

FIG. 1 shows a wireless communication network 100, which may be an LTEnetwork or some other wireless network. Wireless network 100 may includea number of evolved Node Bs (eNBs) 110, 112 and 114 and other networkentities. An eNB may be a station that communicates with the UEs and mayalso be referred to as a base station, a Node B, an access point, etc.Each eNB may provide communication coverage for a particular geographicarea. In 3GPP, the term “cell” can refer to a coverage area of an eNBand/or an eNB subsystem serving this coverage area, depending on thecontext in which the term is used.

An eNB may provide communication coverage for a macro cell, a pico cell,a femto cell, and/or other types of cell. A macro cell may cover arelatively large geographic area (e.g., several kilometers in radius)and may allow unrestricted access by UEs with service subscription. Apico cell may cover a relatively small geographic area and may allowunrestricted access by UEs with service subscription. A femto cell maycover a relatively small geographic area (e.g., a home) and may allowrestricted access by UEs having association with the femto cell (e.g.,UEs in a Closed Subscriber Group (CSG)). An eNB for a macro cell may bereferred to as a macro eNB. An eNB for a pico cell may be referred to asa pico eNB. An eNB for a femto cell may be referred to as a femto eNB ora home eNB (HeNB). In the example shown in FIG. 1, eNB 110 may be amacro eNB for a macro cell 130, eNB 112 may be a femto eNB for a femtocell 132, and eNB 114 may be pico eNB for a pico cell 134. An eNB maysupport one or multiple (e.g., three) cells.

Wireless network 100 may also include relay stations. A relay station isa station that receives a transmission of data from an upstream station(e.g., an eNB or a UE) and sends a transmission of the data to adownstream station (e.g., a UE or an eNB). A relay station may also be aUE that relays transmissions for other UEs. In the example shown in FIG.1, a relay station 116 may communicate with macro eNB 110 and a UE 126in order to facilitate communication between eNB 110 and UE 126. A relaystation may also be referred to as a relay eNB, a relay base station, arelay, etc.

Wireless network 100 may be a heterogeneous network that includes eNBsof different types, e.g., macro eNBs, pico eNBs, femto eNBs, relays,etc. These different types of eNBs may have different transmit powerlevels, different coverage areas, and different impact on interferencein wireless network 100. For example, macro eNBs may have a hightransmit power level (e.g., 20 Watts) whereas pico eNBs, femto eNBs, andrelays may have a lower transmit power level (e.g., 2 Watt).

A network controller 140 may couple to a set of eNBs and may providecoordination and control for these eNBs. Network controller 140 maycommunicate with the eNBs via a backhaul. The eNBs may also communicatewith one another, e.g., directly or indirectly via a wireless orwireline backhaul.

UEs 120 through 128 may be dispersed throughout wireless network 100,and each UE may be stationary or mobile. A UE may also be referred to asa terminal, a mobile station, a subscriber unit, a station, etc. A UEmay be a cellular phone, a personal digital assistant (PDA), a wirelessmodem, a wireless communication device, a handheld device, a laptopcomputer, a cordless phone, a wireless local loop (WLL) station, etc. AUE may be able to communicate with macro eNBs, pico eNBs, femto eNBs,relays, etc. In FIG. 1, a solid line with double arrows indicatescommunication between a UE and a serving eNB, which is an eNB designatedto serve the UE on the downlink and/or uplink. A UE served by a macroeNB may be referred to as a macro UE (MUE). A UE served by a femto eNBmay be referred to as a femto UE or a home UE (HUE).

LTE utilizes orthogonal frequency division multiplexing (OFDM) on thedownlink and single-carrier frequency division multiplexing (SC-FDM) onthe uplink. OFDM and SC-FDM partition a frequency range into multiple(N_(FFT)) orthogonal subcarriers, which are also commonly referred to astones, bins, etc. Each subcarrier may be modulated with data. Ingeneral, modulation symbols are sent in the frequency domain with OFDMand in the time domain with SC-FDM. The spacing between adjacentsubcarriers may be fixed, and the total number of subcarriers (N_(FFT))may be dependent on the system bandwidth. For example, N_(FFT) may beequal to 128, 256, 512, 1024 or 2048 for system bandwidth of 1.25, 2.5,5, 10 or 20 megahertz (MHz), respectively.

The wireless network may utilize frequency division duplexing (FDD) ortime division duplexing (TDD). For FDD, the downlink and uplink may beallocated separate frequency channels, and transmissions may be sentconcurrently on the downlink and uplink via the separate frequencychannels. For TDD, the downlink and uplink may share the same frequencychannel, which may be used for the downlink some of the time and for theuplink some other time.

FIG. 2 shows an exemplary TDD frame structure 200 used in LTE. Thetransmission timeline may be partitioned into units of radio frames.Each radio frame may have a predetermined duration, e.g., 10milliseconds (ms), and may be partitioned into two half-frames. Eachradio frame may also be partitioned into 10 subframes with indices of 0through 9. Each subframe usable for data transmission may be partitionedinto two slots. Each slot may include six symbol periods for an extendedcyclic prefix or seven symbol periods for a normal cyclic prefix. OneOFDMA symbol or one SC-FDMA symbol may be sent in each symbol period.

A number of downlink-uplink configurations are supported by LTE for TDD.Each downlink-uplink configuration indicates whether each subframe is adownlink (DL) subframe used for the downlink, or an uplink (UL) subframeused for the uplink, or a special subframe. Subframes 0 and 5 are usedfor the downlink and subframe 2 is used for the uplink for alldownlink-uplink configurations. Subframes 3, 4, 7, 8 and 9 may each beused for the downlink or uplink depending on the downlink-uplinkconfiguration. Subframe 1 is a special subframe with three specialfields for a downlink pilot time slot (DwPTS), a guard period (GP), andan uplink pilot time slot (UpPTS). Subframe 6 may be (i) a specialsubframe with only the DwPTS or all three special fields or (ii) adownlink subframe, depending on the downlink-uplink configuration. TheDwPTS, GP, and DwPTS fields may have different durations for differentspecial subframe configurations.

The wireless network may support synchronous operation, which may berequired for TDD so that downlink and uplink transmissions to notinterfere with one another and degrade the performance of both thedownlink and uplink. In one design, synchronization may be achieved bysatisfying the following conditions:

-   -   1. A given UE (e.g., a macro UE or a femto UE) receives downlink        signals from all relevant eNBs at the same time (e.g., within a        cyclic prefix),    -   2. A given eNB receives uplink signals from all relevant UEs at        the same time (e.g., within a cyclic prefix),    -   3. No entity (e.g., eNB or UE) has to transmit and receive at        the same time, and    -   4. No UE transmits on a frequency channel at the same time in        which another UE, located in the vicinity, is receiving.

A downlink signal is a signal transmitted by an eNB/base station on thedownlink. A downlink signal may carry a downlink transmission of dataand/or other information. An uplink signal is a signal transmitted by aUE on the uplink. An uplink signal may carry an uplink transmission ofdata and/or other information. Conditions 1 and 2 are applicable forTDD, FDD, and Coordinated MultiPoint (CoMP) transmission. CoMP refers tocoordinate transmission from multiple cells to one or more UEs.Conditions 3 and 4 are applicable for TDD.

In one scheme to satisfy condition 1, all eNBs in the wireless networkmay adjust their transmit timing to be time aligned to a commonreference time, which may be Global Positioning Satellite (GPS) time orsome other time source. For example, each eNB may determine GPS timebased on its GPS receiver and may set its transmit timing to GPS time. Afemto eNB may also measure the receive time of a downlink transmissionfrom a macro eNB, estimate the propagation delay of the downlinktransmission, and advance its transmit timing relative to the measuredreceive time by the propagation delay. The eNBs may also align theirtransmit timing in other manners. In any case, all eNBs may transmitdownlink signals that are aligned in time due to their aligned transmittiming. This scheme may provide good performance in a homogenous networkwith eNBs of only one type, e.g., only macro eNBs. This scheme mayprovide sub-optimal performance in a heterogeneous network with eNBs ofdifferent types, e.g., macro eNBs and femto eNBs, as described below.

In one scheme to satisfy condition 2, all UEs in the wireless networkmay have their transmit timing adjusted by their serving eNBs such thatthe uplink signals from the UEs are time aligned at the serving eNBs.This scheme may provide good performance in a homogenous network but mayprovide sub-optimal performance in a heterogeneous network, as describedbelow.

The schemes described above for adjusting the transmit timing of eNBsand UEs may not provide good performance in a heterogeneous network forseveral reasons. First, the macro eNBs and the femto/pico eNBs typicallyhave substantially different cell sizes (i.e., different size coverageareas), which would result in very different propagation delays in macroand femto/pico cells. Second, the macro eNBs and the femto/pico eNBstypically have substantially different transmit power levels, which mayresult in high interference in certain operating scenarios, as describedbelow.

FIG. 3 shows an exemplary operating scenario with two eNBs and two UEs.In this scenario, macro eNB 110 may communicate with macro UE 120 on thedownlink and uplink, and femto eNB 112 may communicate with femto UE 122on the downlink and uplink. Macro UE 120 may be located far away frommacro eNB 110 and close to femto eNB 112. Macro UE 120 may not be ableto access femto eNB 112 due to restricted association.

UEs 120 and 122 may operate in a dominant interference scenario in theexample shown in FIG. 3. On the downlink, macro UE 120 may observe highinterference from femto eNB 112 due to the close proximity to femto eNB112. On the uplink, macro UE 120 may need to transmit at a high powerlevel in order to reach macro eNB 110 and may then cause highinterference to femto eNB 112. On the downlink, femto UE 122 may observehigh interference from macro eNB 110 due to the higher transmit powerlevel of macro eNB 110. The high interference on the downlink and uplinkmay degrade performance of both UEs 120 and 122.

To mitigate high interference, different time and/or frequency resourcesmay be reserved for UEs 120 and 122. Each UE may then communicate usingits reserved resources, which may be cleared of interference due to theother UE. However, synchronization should be achieved for UEs 120 and122 and femto eNB 112 in order for the reserved resources to mitigateinterference as expected. One reason is because orthogonality betweensubcarriers on different signals transmitted with OFDMA or SC-FDMA maybe maintained if these signals are received within a cyclic prefix. Ifcondition 2 is not satisfied and femto eNB 112 does not receive theuplink signals from UEs 120 and 122 within the cyclic prefix, thenorthogonality between subcarriers on the uplink signals from UEs 120 and122 would not be maintained. In this case, UE 120 may cause interferenceto UE 122 on the uplink even though both UEs transmit on differentresources. Similarly, if condition 1 is not satisfied and femto UE 122does not receive the downlink signals from eNBs 110 and 112 within thecyclic prefix, then orthogonality between subcarriers on the downlinksignals from eNBs 110 and 112 would not be maintained. In this case,macro eNB 110 may cause high interference to femto UE 122 on thedownlink even though eNBs 110 and 112 transmit on different resources toUEs 120 and 122, respectively.

In an aspect, the transmit timing of femto eNB 112 may be delayedrelative to the transmit timing of macro eNB 110 in order to satisfycondition 1 for femto UE 122 and macro UE 120. This timing adjustment isin contrast to the scheme described above in which the transmit timingof all eNBs are aligned, e.g., to GPS time or some other time source.This timing adjustment may improve performance of downlink transmissionto femto UE 122.

In another aspect, the transmit timing of femto UE 122 may be advancedrelative to the transmit timing of femto eNB 112 in order to satisfycondition 2 for femto eNB 112. This timing adjustment is in contrast tothe scheme described above in which the transmit timing of UEs areadjusted to align uplink signals at the serving eNBs. This timingadjustment may improve performance of uplink transmission from femto UE122.

FIG. 4 shows a timing diagram of downlink transmissions from eNBs 110and 112 and uplink transmissions from UEs 120 and 122 in FIG. 3. MacroeNB 110 may send a downlink transmission 410 at time 0, which may bealigned to a reference time such as GPS time. Downlink transmission 410from macro eNB 110 may be received by macro UE 120 at time T_(DIST),which may be the propagation delay between eNB 110 and UE 120. Macro UE120 may have its transmit timing advanced by T_(DIST) relative to thetransmit timing of macro eNB 110. In this case, if macro UE 120 sends anuplink transmission 412 at time −T_(DIST), then this uplink transmissionwould be received by macro eNB 110 at time 0. The transmit timing ofmacro UE 120 may thus be advanced so that the downlink and uplinktransmissions are time aligned at macro eNB 110.

Femto eNB 112 may align its transmit timing with the transmit timing ofmacro eNB 110 and may send a downlink transmission 414 at time 0. Forsimplicity, the femto cell may be assumed to have a very small radius ascompared to the macro cell, and the propagation delay between femto eNB112 and femto UE 122 may be assumed to be negligible. In this case,femto UE 122 may receive downlink transmission 414 from femto eNB 112 attime 0 and may also receive downlink transmission 410 from macro eNB 110at time T_(DIST). Orthogonality between downlink transmissions 410 and414 from eNBs 110 and 112, respectively, may not be maintained withfemto eNB 112 aligning its transmit timing with the transmit timing ofmacro eNB 110.

To satisfy condition 1, femto eNB 112 may delay its transmit timing byT_(DIST) relative to the transmit timing of macro eNB 110. Femto eNB 112may send a downlink transmission 416 at time T_(DIST). Femto UE 122 mayreceive downlink transmission 416 from femto eNB 112 as well as downlinktransmission 410 from macro eNB 110 at time T_(DIST). Orthogonalitybetween downlink transmissions 410 and 416 from eNBs 110 and 112 may bemaintained with femto eNB 112 advancing its transmit timing by T_(DIST)relative to the transmit timing of macro eNB 110.

Femto UE 122 may have its transmit timing delayed by T_(DIST), which maybe twice the negligible propagation delay between femto eNB 112 andfemto UE 122. In this case, if femto UE 122 sends an uplink transmission418 at time T_(DIST), then this uplink transmission would be received byfemto eNB 112 at time T_(DIST). The downlink transmission from femto eNB112 and the uplink transmission from femto UE 122 may then be timealigned at femto eNB 112. However, femto eNB 112 may receive uplinktransmission 412 from macro UE 120 at time −T_(DIST). Orthogonalitybetween uplink transmissions 412 and 418 from UEs 120 and 122,respectively, may not be maintained with the transmit timing of femto UE122 being aligned to the transmit timing of femto eNB 112.

To satisfy condition 2, femto UE 122 may have its transmit timingadvanced by −2T_(DIST) relative to the transmit timing of femto eNB 112.Femto UE 122 may send an uplink transmission 420 at time −T_(DIST).Femto eNB 112 may receive uplink transmission 420 from femto UE 122 aswell as uplink transmission 412 from macro UE 120 at time −T_(DIST).Orthogonality between uplink transmissions 412 and 420 from UEs 120 and122, respectively, may be maintained with femto UE 122 having itstransmit timing advanced by −2T_(DIST) relative to the transmit timingof femto eNB 112.

To satisfy condition 3, the time interval from −T_(DIST) to T_(DIST) maybe defined as a guard interval. This guard interval may be used wheneverthere is a switch from a downlink subframe to an uplink subframe, asdescribed below.

Femto eNB 112 may adjust its transmit timing (e.g., delay its transmittiming by T_(DIST)) in various manners to satisfy condition 1. Femto eNB112 may also adjust the transmit timing of femto UE 122 (e.g., advancethe transmit timing by 2T_(DIST)) in various manners to satisfycondition 2.

In a first design of timing adjustment, femto eNB 112 may implicitly orexplicitly determine T_(DIST) based on a downlink transmission frommacro eNB 110. Femto eNB 112 may receive downlink transmission 410 frommacro eNB 110, e.g., in similar manner as macro UE 120. Femto eNB 112may determine that downlink transmission 410 is received at timeT_(DIST) and may then set its transmit timing equal to T_(DIST)determined by the receive path for macro eNB 110. In this design, thetransmit timing of femto eNB 112 may be advanced relative to thetransmit timing of macro eNB 110 even though the transmit timing ofmacro eNB 110 may not be explicitly determined.

For the first design, femto eNB 112 may assume that downlinktransmission 410 was sent by macro eNB 110 at a reference time, e.g.,GPS time. Femto eNB 112 may then compute T_(DIST) based on the measuredreceive time and the assumed transmit time of downlink transmission 410.Femto eNB 112 may then delay its transmit timing by T_(DIST) from thereference time. Femto eNB 112 may also advance the transmit timing offemto UE 122 by T_(DIST) from the reference time, which would beequivalent to an advance of 2T_(DIST) from the transmit timing of femtoeNB 112.

In a second design, femto eNB 112 may determine T_(DIST) based on anuplink transmission from macro UE 120. Femto eNB 112 may measure uplinktransmission 412 from macro UE 120 and determine the receive time ofuplink transmission 412, which may be approximately equal to thetransmit time of uplink transmission 412 because of the close proximitybetween macro UE 120 and femto eNB 112. Femto eNB 112 may assume thatthe transmit timing of macro UE 120 has been advanced by T_(DIST)relative to the reference time. Femto eNB 112 may compute T_(DIST) basedon the difference between the receive time of uplink transmission 412and the reference time. Femto eNB 112 may then delay its transmit timingby T_(DIST) relative to the reference time and may also advance thetransmit timing of femto UE 122 by T_(DIST) from the reference time.

In a third design, femto eNB 112 may determine T_(DIST) based onlocation information. Femto eNB 112 may obtain its location, e.g., byperforming GPS positioning. Femto eNB 112 may also obtain the locationof macro eNB 110, e.g., from system information broadcast by eNB 110 orsent via the backhaul. Femto eNB 112 may compute the distance betweeneNBs 110 and 112 based on their known locations and may convert thecomputed distance to a propagation delay of T_(DIST). Femto eNB 112 mayassume that macro eNB 110 aligned its transmit timing to the referencetime. Femto eNB 112 may then delay its transmit timing by T_(DIST)relative to the reference time. Femto eNB 112 may also advance thetransmit timing of femto UE 122 by T_(DIST) from the reference time

In a fourth design, femto eNB 112 may determine T_(DIST) based onone-way ranging with macro eNB 110. Femto eNB 112 may send atransmission at a known time (e.g., aligned to GPS time) to macro eNB110. Macro eNB 110 may receive the transmission from femto eNB 112 andmay measure the propagation delay of T_(DIST) based on the measuredreceive time and the known transmit time of the transmission from femtoeNB 112. Macro eNB 110 may then send a message with the measuredpropagation delay to femto eNB 112. Femto eNB 112 may adjust itstransmit timing as well as the transmit timing of femto UE 122 based onthe measured propagation delay from macro eNB 110.

In a fifth design, femto eNB 112 may determine T_(DIST) based on two-wayranging with macro eNB 110. Femto eNB 112 may send a transmission tomacro eNB 110, which may return the transmission back to femto eNB 112.Femto eNB 112 may measure a round trip delay (RTD) based on the receivetime of the return transmission from macro eNB 110 and the transmit timeof the original transmission from femto eNB 112. Femto eNB 112 maycompute T_(DIST) to be one half of the RTD. Femto eNB 112 may thenadjust its transmit timing as well as the transmit timing of femto UE122 based on the computed T_(DIST).

In a sixth design, femto eNB 112 may determine T_(DIST) based on rangingby UE 120 or 122 with macro eNB 110. UE 120 or 122 may perform one-wayranging with macro eNB 110 (e.g., as described above for the fourthdesign) and may obtain T_(DIST) from the one-way ranging. UE 120 or 122may also perform two-way ranging with macro eNB 110 (e.g., as describedabove for the fifth design) and may obtain RTD from the two-way ranging.UE 120 or 122 may then send the measured T_(DIST) or RTD (directly orindirectly) to femto eNB 112. For example, femto UE 122 may send themeasured T_(DIST) or RTD directly to femto eNB 112. Macro UE 120 maysend the measured T_(DIST) or RTD to macro eNB 110, which may thenforward the measured T_(DIST) or RTD to femto eNB 112 via the backhaulor over the air. In any case, femto eNB 112 may delay its transmittiming and may advance the transmit timing of femto UE 122 based on themeasured T_(DIST) or RTD.

Ranging by UE 120 or 122 with macro eNB 110 may be initiated andperformed in various manners. In one design, femto eNB 112 may requestUE 120 or 122 to perform ranging with macro eNB 110 and to send theT_(DIST) or RTD to femto eNB 112. In another design, femto eNB 112 maycommunicate with macro eNB 110, e.g., via a backhaul or over the air.Macro eNB 110 may then request UE 120 or 122 to perform ranging withmacro eNB 110.

In a seventh design, femto eNB 112 may determine T_(DIST) based onranging by macro eNB 110 with UE 120 or 122. Macro eNB 110 may performone-way ranging with macro UE 120 (e.g., as described above for thefourth design) and may obtain T_(DIST) from the one-way ranging. MacroeNB 110 may also perform two-way ranging with macro UE 120 (e.g., asdescribed above for the fifth design) and may obtain RTD from thetwo-way ranging. Macro eNB 110 may then send the measured T_(DIST) orRTD (directly or indirectly) to femto eNB 112. Femto eNB 112 may thendelay its transmit timing and may advance the transmit timing of femtoUE 122 based on the measured T_(DIST) or RTD.

Ranging by macro eNB 110 may be initiated and performed in variousmanners. In one design, femto eNB 112 may request macro eNB 110 toperform ranging with UE 120 or 122. In another design, femto eNB 112 mayrequest UE 120 or 122 to ask macro eNB 110 to perform ranging with UE120 or 122.

In an eight design, femto eNB 112 may determine T_(DIST) based on acoarse approximation. For example, femto eNB 112 may obtain a coarseestimate of T_(DIST) based on a reference signal received power (RSRP)on the downlink for macro eNB 110. Since macro eNB 110 broadcastsinformation about its transmit power for the reference signal, femto eNB112 can estimate signal propagation pathloss as a ratio of the transmitpower of macro eNB 110 to the RSRP measured by femto eNB 112. Theestimated pathloss may be used to estimate the distance between macroeNB 110 and femto eNB 112.

Femto eNB 112 may also determine T_(DIST) in other manners. In onedesign, T_(DIST) may be determined when femto eNB 112 is powered up andmay be used to adjust the transmit timing of femto eNB 112. T_(DIST) maybe fairly constant for femto eNB 112, and the transmit timing of femtoeNB 112 may not change much once it is set properly. T_(DIST) may bedetermined again if necessary, e.g., if femto eNB 112 is moved to a newlocation, which may be sufficiently far from the prior location.

FIG. 5 shows a timing diagram for femto eNB 112 and femto UE 122 withtiming adjustment to satisfy conditions 1 and 2 for TDD. Femto eNB 112may have its transmit timing delayed by T_(DIST) relative to thereference time, which may be GPS time, or the transmit timing of macroeNB 110, or some other time. From the perspective of femto eNB 112,subframe 1 may start at time T_(1D), subframe 2 may start at timeT_(2D), etc. Femto UE 122 may have its transmit timing advanced byT_(DIST) relative to the reference time. From the perspective of femtoUE 122, subframe 1 may start at time T_(1U), subframe 2 may start attime T_(2U), etc. Time T^(1U) may be earlier than time T_(1D) by2T_(DIST).

Femto eNB 112 may send a downlink transmission to femto UE 122 insubframe 1. A switch from downlink to uplink may occur after subframe 1.Femto UE 122 may then send an uplink transmission to femto eNB 112 insubframe 2. Subframe 2 on the uplink may overlap subframe 1 on thedownlink from time T_(2U) to time T_(2D). A guard interval may bedefined from time T_(2U) to time T_(2D) and may have a duration of2T_(DIST). Femto UE 122 may avoid transmitting during the guard intervalin order to avoid interfering with the downlink transmission from femtoeNB 112. In general, the first uplink subframe following a downlinksubframe may be reduced by 2T_(DIST) due to the guard interval.

In one design, a guard interval from −T_(DIST,MAX) to T_(DIST,MAX)centered at the reference time may be used, where T_(DIST,MAX) may bedetermined based on the cell size of macro eNB 110. This design mayensure that conditions 3 and 4 can be satisfied regardless of wherefemto eNB 112 is located relative to macro eNB 110.

In one design, the guard interval from −T_(DIST,MAX) to T_(DIST,MAX) orfrom −T_(DIST) to T_(DIST) may be enabled when femto eNB 112 is withinthe coverage of macro eNB 110. The guard interval may be reduced to asmaller value (e.g., zero) when femto eNB 112 is not within the coverageof macro eNB 110. This design may avoid a reduction in the size of anuplink subframe following a downlink subframe when the reduction is notnecessary.

The transmit timing of femto UE 122 may be advanced in various manners.In one design, femto UE 122 may have its transmit timing advanced byfemto eNB 112. Femto UE 122 may send a random access probe when it iswithin the coverage of femto eNB 112. Femto eNB 112 may receive therandom access probe from femto UE 122 and may determine the currenttransmit timing of femto UE 122 based on the receive time of the randomaccess probe. Femto eNB 112 may determine the desired transmit timing offemto UE 122 to be 2T_(DIST) earlier than the transmit timing of femtoeNB 112. Femto eNB 112 may then determine a timing adjustment for femtoUE 122 based on the difference between the current transmit timing andthe desired transmit timing of femto UE 122. Femto eNB 112 may send arandom access response with the timing adjustment to femto UE 122. FemtoeNB 112 may also periodically determine the transmit timing of femto UE122 (e.g., every X seconds) and may send relative commands to change(e.g., to advance or delay) the transmit timing of femto UE 122 asnecessary.

The transmit timing of femto UE 122 may also be adjusted in othermanners. For example, femto UE 122 may determine the propagation delayT_(DIST) using any of the designs described above and may set itstransmit timing based on the propagation delay. Femto UE 122 may alsomeasure an uplink transmission from macro UE 120 and may set itstransmit timing based on the receive time of the uplink transmissionfrom macro UE 120.

Femto eNB 112 may have a synchronization requirement of T_(REQ), whichmay be given as T_(REQ)=T_(DIST)+T_(MARGIN), where T_(MARGIN) may be animplementation margin. The synchronization requirement of femto eNB 112may be satisfied by delaying the transmit timing of femto eNB 112 byT_(DIST), as shown in FIG. 5.

FIG. 6 shows a design of a process 600 for adjusting transmit timing ofa base station/eNB in a wireless network. Process 600 may be performedby a base station or by some other entity. The presence of a first basestation may be detected by a second base station (block 612). Thetransmit timing of the second base station may be delayed relative tothe transmit timing of the first base station (block 614).

In one design, a UE located within the coverage of the first and secondbase stations may be identified. The transmit timing of the second basestation may be delayed relative to the transmit timing of the first basestation to time align downlink signals from the first and second basestations at the UE. In another design, the transmit timing of the secondbase station may be delayed by a predetermined amount, which may beequal to the propagation delay (e.g., T_(DIST,MAX)) correspond to thecell size of the first base station or some other amount.

In one design, the first base station may be a macro base station andthe second base station may be a femto base station. The UE may be afemto UE communicating with the femto base station or a macro UEcommunicating the macro base station. The UE may be located closer tothe femto base station than the macro base station. In other designs,the first and second base stations may be other types of base stations.For example, the second base station may be a pico base station, a relaystation, etc.

FIG. 7 shows a design of block 614 in FIG. 6. The propagation delaybetween the first base station and the second base station may bedetermined (block 712). The amount of delay for the transmit timing ofthe second base station may be determined based on the propagation delay(block 714). A reference time (e.g., GPS time) used for the transmittiming of the first base station may be determined (block 716). Thetransmit timing of the second base station may then be delayed by thedetermined amount of delay from the reference time (block 718).

In one design of block 712, the propagation delay between the first andsecond base stations may be determined based on the location of thefirst base station and the location of the second base station. Inanother design of block 712, the propagation delay between the first andsecond base stations may be determined based on ranging between thefirst and second base stations. In yet another design of block 712, thepropagation delay between the first base station and a UE within thecoverage of the second base station may be determined based on rangingbetween the first base station and the UE. The propagation delay mayalso be determined in other manners.

Referring back to FIG. 6, in another design of block 614, the secondbase station may receive a downlink transmission from the first basestation. The second base station may then determine its transmit timingbased on (e.g., may align its transmit timing to) the receive time ofthe downlink transmission from the first base station.

In yet another design of block 614, the second base station may receivean uplink transmission from a UE communicating with the first basestation. The UE may have its transmit timing advanced relative to thetransmit timing of the first base station. The second base station maydetermine its transmit timing based on the receive time of the uplinktransmission from the UE and the reference time.

FIG. 8 shows a design of an apparatus 800 for adjusting transmit timingof a base station in a wireless network. Apparatus 800 includes a module812 to detect the presence of a first base station by a second basestation, and a module 814 delay the transmit timing of the second basestation relative to the transmit timing of the first base station.

FIG. 9 shows a design of a process 900 for adjusting transmit timing ofa UE in a wireless network. Process 900 may be performed by a first basestation (as described below) or by some other entity. The first basestation may identify a first UE located within the coverage of the firstbase station (block 912). The first base station may advance thetransmit timing of the first UE relative to the transmit timing of thefirst base station by an amount larger than twice the propagation delaybetween the first UE and the first base station (block 914).

In one design, the first base station may advance the transmit timing ofthe first UE to time align uplink signals from the first UE and a secondUE at the first base station. The first UE may communicate with thefirst base station, and the second UE may communicate with a second basestation. The first base station may be a femto base station and thesecond base station may be a macro base station. The first and secondUEs may be located closer to the femto base station than the macro basestation. In other designs, the first and second base stations and thefirst and second UEs may be other types of base stations and UEs.

FIG. 10 shows a design of block 914 in FIG. 9. The first base stationmay determine second propagation delay between the second base stationand the first UE, or the second UE, or the first base station, e.g.,based on any of the designs described above (block 1012). The first basestation may determine the amount of advance for the transmit timing ofthe first UE based on the second propagation delay (block 1014). Thefirst base station may determine a reference time (e.g., GPS time) usedfor the transmit timing of the second base station (block 1016). Thefirst base station may then advance the transmit timing of the first UEby the determined amount of advance from the reference time (block1018).

Referring back to FIG. 9, in another design of block 914, the first basestation may receive an uplink transmission from the second UE. The firstbase station may then determine the transmit timing of the first UEbased on (e.g., may align the transmit timing of the first UE to) thereceive time of the uplink transmission from the second UE.

In one design, the first base station may receive a random access probefrom the first UE and may identify the first UE based on the randomaccess probe. The first base station may determine a timing adjustmentfor the first UE to time align the uplink signals from the first andsecond UEs at the first base station. The first base station may thensend the timing adjustment to the first UE, e.g., in response to therandom access probe.

In one design, the first base station may delay its transmit timingrelative to the transmit timing of the second base station, e.g., totime align downlink signals from the first and second base stations atthe first and second UEs (block 916). The transmit timing of the firstbase station may be delayed by a particular amount and the transmittiming of the first UE may be advanced by the same amount relative tothe transmit timing of the second base station, e.g., as shown in FIG.5.

In one design, the first base station may determine a guard interval ofat least the difference between its transmit timing and the transmittiming of the first UE (block 918). In one design, the guard intervalmay be set to a predetermined value (e.g., T_(DIST,MAX)), which may bedetermined based on the cell size of the second base station. The guardinterval may be used for an uplink transmission from the first UEfollowing a downlink transmission from the first base station, e.g., asshown in FIG. 5 (block 920).

FIG. 11 shows a design of an apparatus 1100 for adjusting transmittiming of a UE in a wireless network. Apparatus 1100 includes a module1112 to identify a UE located within the coverage of a first basestation, a module 1114 to advance transmit timing of the UE relative totransmit timing of the first base station by an amount larger than twicea propagation delay between the UE and the first base station, a module1116 to delay the transmit timing of the first base station relative totransmit timing of a second base station, a module 1118 to determine aguard interval of at least the difference between the transmit timing ofthe first base station and the transmit timing of the UE, and a module1120 to use the guard interval for an uplink transmission from the UEfollowing a downlink transmission from the first base station.

FIG. 12 shows a design of a process 1200 for communication in a wirelessnetwork. Process 1200 may be performed by a first UE (as describedbelow) or by some other entity. The first UE may receive a timingadjustment to advance its transmit timing relative to the transmittiming of a first base station by an amount larger than twice thepropagation delay between the first UE and the first base station (block1212). The first UE may send an uplink transmission to the first basestation based on the transmit timing of the first UE (block 1214).

In one design, the transmit timing of the first UE may be advanced totime align uplink signals from the first UE and a second UE at the firstbase station. The first UE may be located within the coverage of thefirst base station and a second base station. In one design, the firstbase station may be a femto base station and the second base station maybe a macro base station. The first UE may communicate with the femtobase station, and the second UE may communicate with the macro basestation. The first and second UEs may be located closer to the femtobase station than the macro base station. In other designs, the firstand second base stations and the first and second UEs may be other typesof base stations and UEs.

In one design, the first UE may send a random access probe and mayreceive the timing adjustment from the first base station in response tothe random access probe. The timing adjustment may also be sent to thefirst UE in response to some other action.

In one design, the first base station may determine the timingadjustment without any interaction with the first UE. In another design,the first UE may perform ranging with the second base station todetermine a second propagation delay between the second base station andthe first UE. The transmit timing of the first UE may then be advancedby an amount determined based on the second propagation delay.

FIG. 13 shows a design of an apparatus 1300 for communication in awireless network. Apparatus 1300 includes a module 1312 to receive atiming adjustment to advance the transmit timing of a UE relative to thetransmit timing of a base station by an amount larger than twice thepropagation delay between the UE and the base station, and a module 1314to send an uplink transmission from the UE to the base station based onthe transmit timing of the UE.

The modules in FIGS. 8, 11 and 13 may comprise processors, electronicdevices, hardware devices, electronic components, logical circuits,memories, software codes, firmware codes, etc., or any combinationthereof.

FIG. 14 shows a block diagram of a design of base station/eNB 112 and UE122 in FIGS. 1 and 3. Base station 112 may be equipped with T antennas1434 a through 1434 t, and UE 122 may be equipped with R antennas 1452 athrough 1452 r, where in general T≧1 and R≧1.

At base station 112, a transmit processor 1420 may receive data for oneor more UEs from a data source 1412, process (e.g., encode and modulate)the data for each UE based on one or more modulation and coding schemes,and provide data symbols for all UEs. Transmit processor 1420 may alsoreceive control information (e.g., timing adjustment for UE 122) from acontroller/processor 1440, process the control information, and providecontrol symbols. Transmit processor 1420 may also generate referencesymbols for a reference signal or pilot. A transmit (TX) multiple-inputmultiple-output (MIMO) processor 1430 may perform spatial processing(e.g., precoding) on the data symbols, the control symbols, and/or thereference symbols, if applicable, and may provide T output symbolstreams to T modulators (MOD) 1432 a through 1432 t. Each modulator 1432may process a respective output symbol stream (e.g., for OFDM, etc.) toobtain an output sample stream. Each modulator 1432 may further process(e.g., convert to analog, amplify, filter, and upconvert) the outputsample stream to obtain a downlink signal. T downlink signals frommodulators 1432 a through 1432 t may be transmitted via T antennas 1434a through 1434 t, respectively.

At UE 122, R antennas 1452 a through 1452 r may receive the downlinksignals from base station 112 and possibly other base stations and mayprovide received signals to demodulators (DEMOD) 1454 a through 1454 r,respectively. Each demodulator 1454 may condition (e.g., filter,amplify, downconvert, and digitize) its received signal to obtainreceived samples and may further process the received samples (e.g., forOFDM, etc.) to obtain received symbols. A MIMO detector 1460 may performMIMO detection (if applicable) on the received symbols from all Rdemodulators 1454 a through 1454 r and provide detected symbols. Areceive processor 1470 may process (e.g., demodulate and decode) thedetected symbols, provide decoded data for UE 122 to a data sink 1472,and provide decoded control information to a controller/processor 1490.

On the uplink, at UE 122, data from a data source 1478 and controlinformation (e.g., random access probe) from controller/processor 1490may be processed by a transmit processor 1480, precoded by a TX MIMOprocessor 1482 (if applicable), conditioned by modulators 1454 a through1454 r, and transmitted via antennas 1452 a through 1452 r. At basestation 112, the uplink signals from UE 122 and other UEs may bereceived by antennas 1434, conditioned by demodulators 1432, detected bya MIMO detector 1436, and processed by a receive processor 1438 toobtain the data and control information transmitted by UE 122 and otherUEs. Processor 1438 may provide the recovered data to a data sink 1439and the recovered control information to controller/processor 1440.

Controllers/processors 1440 and 1490 may direct the operation at basestation 112 and UE 122, respectively. A channel processor 1494 mayreceive downlink transmissions from base stations and determine thereceive time of the downlink transmissions. A channel processor 1446 mayreceive transmissions from UEs and possibly other base stations anddetermine the receive time of the transmissions. Processor 1440 and/orother processors and modules at base station 112 may perform or directprocess 600 in FIG. 6, process 614 in FIG. 7, process 900 in FIG. 9,process 914 in FIG. 10, and/or other processes for the techniquesdescribed herein. Processor 1490 and/or other processors and modules atUE 122 may perform or direct process 1200 in FIG. 12 and/or otherprocesses for the techniques described herein. Memories 1442 and 1492may store data and program codes for base station 112 and UE 122,respectively. A scheduler 1444 may schedule UEs for transmission on thedownlink and/or uplink and may assign resources to the scheduled UEs.

Those of skill in the art would understand that information and signalsmay be represented using any of a variety of different technologies andtechniques. For example, data, instructions, commands, information,signals, bits, symbols, and chips that may be referenced throughout theabove description may be represented by voltages, currents,electromagnetic waves, magnetic fields or particles, optical fields orparticles, or any combination thereof.

Those of skill would further appreciate that the various illustrativelogical blocks, modules, circuits, and algorithm steps described inconnection with the disclosure herein may be implemented as electronichardware, computer software, or combinations of both. To clearlyillustrate this interchangeability of hardware and software, variousillustrative components, blocks, modules, circuits, and steps have beendescribed above generally in terms of their functionality. Whether suchfunctionality is implemented as hardware or software depends upon theparticular application and design constraints imposed on the overallsystem. Skilled artisans may implement the described functionality invarying ways for each particular application, but such implementationdecisions should not be interpreted as causing a departure from thescope of the present disclosure.

The various illustrative logical blocks, modules, and circuits describedin connection with the disclosure herein may be implemented or performedwith a general-purpose processor, a digital signal processor (DSP), anapplication specific integrated circuit (ASIC), a field programmablegate array (FPGA) or other programmable logic device, discrete gate ortransistor logic, discrete hardware components, or any combinationthereof designed to perform the functions described herein. Ageneral-purpose processor may be a microprocessor, but in thealternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

The steps of a method or algorithm described in connection with thedisclosure herein may be embodied directly in hardware, in a softwaremodule executed by a processor, or in a combination of the two. Asoftware module may reside in RAM memory, flash memory, ROM memory,EPROM memory, EEPROM memory, registers, hard disk, a removable disk, aCD-ROM, or any other form of storage medium known in the art. Anexemplary storage medium is coupled to the processor such that theprocessor can read information from, and write information to, thestorage medium. In the alternative, the storage medium may be integralto the processor. The processor and the storage medium may reside in anASIC. The ASIC may reside in a user terminal. In the alternative, theprocessor and the storage medium may reside as discrete components in auser terminal.

In one or more exemplary designs, the functions described may beimplemented in hardware, software, firmware, or any combination thereof.If implemented in software, the functions may be stored on ortransmitted over as one or more instructions or code on acomputer-readable medium. Computer-readable media includes both computerstorage media and communication media including any medium thatfacilitates transfer of a computer program from one place to another. Astorage media may be any available media that can be accessed by ageneral purpose or special purpose computer. By way of example, and notlimitation, such computer-readable media can comprise RAM, ROM, EEPROM,CD-ROM or other optical disk storage, magnetic disk storage or othermagnetic storage devices, or any other medium that can be used to carryor store desired program code means in the form of instructions or datastructures and that can be accessed by a general-purpose orspecial-purpose computer, or a general-purpose or special-purposeprocessor. Also, any connection is properly termed a computer-readablemedium. For example, if the software is transmitted from a website,server, or other remote source using a coaxial cable, fiber optic cable,twisted pair, digital subscriber line (DSL), or wireless technologiessuch as infrared, radio, and microwave, then the coaxial cable, fiberoptic cable, twisted pair, DSL, or wireless technologies such asinfrared, radio, and microwave are included in the definition of medium.Disk and disc, as used herein, includes compact disc (CD), laser disc,optical disc, digital versatile disc (DVD), floppy disk and blu-ray discwhere disks usually reproduce data magnetically, while discs reproducedata optically with lasers. Combinations of the above should also beincluded within the scope of computer-readable media.

The previous description of the disclosure is provided to enable anyperson skilled in the art to make or use the disclosure. Variousmodifications to the disclosure will be readily apparent to thoseskilled in the art, and the generic principles defined herein may beapplied to other variations without departing from the spirit or scopeof the disclosure. Thus, the disclosure is not intended to be limited tothe examples and designs described herein but is to be accorded thewidest scope consistent with the principles and novel features disclosedherein.

What is claimed is:
 1. A method for wireless communication, comprising:identifying a first user equipment (UE) located within coverage of afirst base station; and advancing transmit timing of the first UErelative to transmit timing of the first base station by an amountlarger than twice a propagation delay between the first UE and the firstbase station.
 2. The method of claim 1, wherein the advancing thetransmit timing of the first UE comprises advancing the transmit timingof the first UE to time align uplink signals from the first UE and asecond UE at the first base station, the second UE communicating with asecond base station.
 3. The method of claim 2, wherein the first basestation is a femto base station and the second base station is a macrobase station, and wherein the first UE communicates with the femto basestation.
 4. The method of claim 2, wherein the advancing the transmittiming of the first UE comprises determining a second propagation delaybetween the second base station and the first UE, or the second UE, orthe first base station, and determining an amount of advance for thetransmit timing of the first UE based on the second propagation delay.5. The method of claim 2, wherein the advancing the transmit timing ofthe first UE comprises receiving an uplink transmission from the secondUE at the first base station, and determining the transmit timing of thefirst UE based on receive time of the uplink transmission from thesecond UE at the first base station.
 6. The method of claim 2, whereinthe advancing the transmit timing of the first UE comprises determininga timing adjustment for the first UE to time align the uplink signalsfrom the first and second UEs at the first base station, and sending thetiming adjustment to the first UE.
 7. The method of claim 6, wherein theidentifying the first UE comprises receiving a random access probe fromthe first UE at the first base station, and wherein the timingadjustment is determined and sent to the first UE in response to therandom access probe.
 8. The method of claim 1, further comprising:delaying the transmit timing of the first base station relative totransmit timing of a second base station.
 9. The method of claim 8,wherein the transmit timing of the first base station is delayed by aparticular amount and the transmit timing of the first UE is advanced bythe particular amount relative to the transmit timing of the second basestation.
 10. The method of claim 1, further comprising: determining aguard interval of at least a difference between the transmit timing ofthe first base station and the transmit timing of the first UE; andusing the guard interval for an uplink transmission from the first UEfollowing a downlink transmission from the first base station.
 11. Themethod of claim 10, wherein the guard interval is set to a valuedetermined based on a cell size of a second base station.
 12. Anapparatus for wireless communication, comprising: means for identifyinga first user equipment (UE) located within coverage of a first basestation; and means for advancing transmit timing of the first UErelative to transmit timing of the first base station by an amountlarger than twice a propagation delay between the first UE and the firstbase station.
 13. The apparatus of claim 12, wherein the means foradvancing the transmit timing of the first UE comprises means foradvancing the transmit timing of the first UE to time align uplinksignals from the first UE and a second UE at the first base station, thesecond UE communicating with a second base station.
 14. The apparatus ofclaim 13, wherein the means for advancing the transmit timing of thefirst UE comprises means for determining a second propagation delaybetween the second base station and the first UE, or the second UE, orthe first base station, and means for determining an amount of advancefor the transmit timing of the first UE based on the second propagationdelay.
 15. The apparatus of claim 13, wherein the means for advancingthe transmit timing of the first UE comprises means for receiving anuplink transmission from the second UE at the first base station, andmeans for determining the transmit timing of the first UE based onreceive time of the uplink transmission from the second UE at the firstbase station.
 16. The apparatus of claim 13, wherein the means foradvancing the transmit timing of the first UE comprises means fordetermining a timing adjustment for the first UE to time align theuplink signals from the first and second UEs at the first base station,and means for sending the timing adjustment to the first UE.
 17. Theapparatus of claim 13, further comprising: means for delaying thetransmit timing of the first base station relative to transmit timing ofthe second base station.
 18. The apparatus of claim 12, furthercomprising: means for determining a guard interval of at least adifference between the transmit timing of the first base station and thetransmit timing of the first UE; and means for using the guard intervalfor an uplink transmission from the first UE following a downlinktransmission from the first base station.
 19. An apparatus for wirelesscommunication, comprising: at least one processor configured to identifya user equipment (UE) located within coverage of a base station, and toadvance transmit timing of the UE relative to transmit timing of thebase station by an amount larger than twice a propagation delay betweenthe UE and the base station.
 20. A computer program product, comprising:a computer-readable medium comprising: code for causing at least onecomputer to identify a user equipment (UE) located within coverage of abase station, and code for causing the at least one computer to advancetransmit timing of the UE relative to transmit timing of the basestation by an amount larger than twice a propagation delay between theUE and the base station.
 21. A method for wireless communication,comprising: receiving a timing adjustment to advance transmit timing ofa first user equipment (UE) relative to transmit timing of a first basestation by an amount larger than twice a propagation delay between thefirst UE and the first base station; and sending an uplink transmissionfrom the first UE to the first base station based on the transmit timingof the first UE.
 22. The method of claim 21, further comprising: sendinga random access probe from the first UE; and receiving the timingadjustment from the first base station in response to the random accessprobe.
 23. The method of claim 21, wherein the transmit timing of thefirst UE is advanced to time align uplink signals from the first UE anda second UE at the first base station, the first UE being located withincoverage of the first base station and a second base station, and thesecond UE communicating with the second base station.
 24. The method ofclaim 23, further comprising: performing ranging between the first UEand the second base station to determine a second propagation delaybetween the second base station and the first UE, wherein the transmittiming of the first UE is advanced by an amount determined based on thesecond propagation delay.
 25. An apparatus for wireless communication,comprising: means for receiving a timing adjustment to advance transmittiming of a first user equipment (UE) relative to transmit timing of afirst base station by an amount larger than twice a propagation delaybetween the first UE and the first base station; and means for sendingan uplink transmission from the first UE to the first base station basedon the transmit timing of the first UE.
 26. The apparatus of claim 25,further comprising: means for sending a random access probe from thefirst UE; and means for receiving the timing adjustment from the firstbase station in response to the random access probe.
 27. The apparatusof claim 25, wherein the transmit timing of the first UE is advanced totime align uplink signals from the first UE and a second UE at the firstbase station, the first UE being located within coverage of the firstbase station and a second base station, and the second UE communicatingwith the second base station.
 28. The apparatus of claim 27, furthercomprising: means for performing ranging between the first UE and thesecond base station to determine a second propagation delay between thesecond base station and the first UE, wherein the transmit timing of thefirst UE is advanced by an amount determined based on the secondpropagation delay.
 29. An apparatus for wireless communication,comprising: at least one processor configured to receive a timingadjustment to advance transmit timing of a user equipment (UE) relativeto transmit timing of a base station by an amount larger than twice apropagation delay between the UE and the base station, and to send anuplink transmission from the UE to the base station based on thetransmit timing of the UE.
 30. A computer program product, comprising: acomputer-readable medium comprising: code for causing at least onecomputer to receive a timing adjustment to advance transmit timing of auser equipment (UE) relative to transmit timing of a base station by anamount larger than twice a propagation delay between the UE and the basestation, and code for causing the at least one computer to send anuplink transmission from the UE to the base station based on thetransmit timing of the UE.